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O-MBR
International Biodeterioration & Biodegradation 144 (2019) 104755
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Controlling sludge retention time to alleviate inhibition of nitrosation and nitration by accumulated aluminum in an A/O-MBR
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Chenggang Qiu1, Chuanhe Yang1, Chunhua He, Miao Gong, Zhenhu Hu, Wei Wang* Department of Municipal Engineering, School of Civil Engineering, Hefei University of Technology, Hefei, 230009, China
ARTICLE INFO
ABSTRACT
Keywords: Sludge retention time Poly-aluminum chloride Anoxic-oxic membrane bioreactor Biological phosphorus removal Nitrification Inhibition
Severe membrane fouling and limited effect of biological phosphorus removal are the major drawbacks for anoxicoxic membrane bioreactor (A/O-MBR). The dosing of poly-aluminum chloride in A/O-MBR can reduce membrane fouling and improve phosphorus removal certainly, but it may bring potential impacts on the specific oxygen uptake rate of sludge and the activity of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria with accumulated aluminum. Controlling sludge retention time (SRT) was firstly investigated to evaluate the effect on alleviating inhibition caused by accumulated aluminum in A/O-MBR. As compared to the performance without dosing of poly-aluminum chloride, the effluent ammonia concentration in the SRT-15d reactor was increased by 91.80% lower than that of SRT-30d reactor (111.84%). Due to higher sludge yield rate in the SRT-15d reactor, the amount of accumulated aluminum was 39.33 mg Al g−1 VSS less than that of the SRT-30d reactor. Besides, the decline of the activity of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the SRT-15d reactor was 4.4% and 18.3%, respectively lower than that of the SRT-30d reactor. Therefore, controlling shorter SRT can be used as an effective tool to alleviate inhibition of nitration and nitrosation by accumulated aluminum in A/O-MBR.
1. Introduction Anoxic-oxic membrane bioreactor (A/O-MBR) is a promising technology for wastewater treatment due to its advantages of reduced footprint, low sludge yield, and high quality of effluent (She et al., 2016). However, the membrane fouling is an inevitable problem (Kochkodan and Hilal, 2015). Meanwhile, with complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), the A/ O-MBR shows a weakness of biological phosphorus removal in a long SRT operation (Falahti-Marvast and Karimi-Jashni, 2015; Xiao et al., 2019). In general, some chemical coagulants such as polyferric chloride (Li et al., 2018; Yang et al., 2011) and polyaluminium chloride (Yu et al., 2015) were employed to simultaneously alleviate membrane fouling and enhance phosphorus removal. Nevertheless, it might bring potential impact on the microorganisms by the accumulated metal salts in the A/O-MBR. Previous studies aimed at the effect of chemical coagulants on reducing membrane fouling and achieving excellent phosphorus removal, ignoring the accumulation of metal salts, especially for aluminum salts in the A/O-MBR with a long SRT (Park et al., 2018; Zhang et al., 2015). Until recent years, it was reported that the trivalent cations such like Al3+ could accelerate lipid oxidation on the bacterial membrane
(Londono et al., 2017) and affect the bacterial activity of sludge such as ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) (Liu et al., 2011). The polymeric aluminum salt covering on the sludge would lead to both changes of sludge morphology and properties to inhibit the microbial metabolism (Cao et al., 2016). Even worse, the toxicity of aluminum salts to cells was expressed as damaging cell wall, interrupting the transport of molecules across cell membranes and impairing intercellular process (Riaz et al., 2018). The inhibition of accumulated aluminum salts on the microorganisms was more easily ignored in wastewater treatment. In fact, Liu et al. (2011) reviewed that the accumulated aluminum salt had an inhibitory effect on the bacterial activity with long-term dosing of aluminum salt. Despite the phenomena of PAC inhibition on the microorganisms have been found in the past, the accumulation rate and control method of aluminum in sludge has not been documented before. The aims of this study were to investigate the relationship between the accumulated aluminum salt and the inhibition of specific oxygen uptake rate of sludge. Furthermore, the performance of A/O-MBR without and with dosing of PAC was compared at different SRTs. Finally, the mechanism of controlling SRT to alleviate the inhibition of accumulated aluminum in the A/O-MBR was discussed.
*
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Wang). 1 Co-first author. https://doi.org/10.1016/j.ibiod.2019.104755 Received 22 May 2019; Received in revised form 23 July 2019; Accepted 5 August 2019 0964-8305/ © 2019 Elsevier Ltd. All rights reserved.
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L−1. Ammonia chloride (30 mg NH4+-N L−1) was dosed into the sealed beakers. The liquid samples were collected and filtered with 0.45 μm filters to measure the concentration of NH4+-N. The AOR was calculated as formula (2). Similar to the AOR tests, the NOR tests were conducted by dosing of sodium nitrite (30 mg NO2−-N L−1) in the sealed beakers and measuring the decreasing slope of NO2−-N concentration as formula (3).
2. Materials and methods 2.1. Experimental setup and operating conditions The reactor was made up of an A/O reactor and a submerged hollow fiber polyvinylidene fluoride (PVDF) microfiltration membrane module with the 0.2 μm of pore size and 15 L m−2 h−1 of membrane flux (as shown in Fig. S1). The reactor with a 6.1 L of working volume was operated at a 12 h of HRT and room temperature. Two pumps were employed to circulate wastewater and sludge. Before dosing PAC, the pH in the effluent was at the range of 7.4–8.4, while it slightly decreased to 7.3–8.1 after dosing of PAC. Besides, the concentration of DO in the aerobic zone was controlled at the range of 2–4 mg O2 L−1 during the whole operation. The membrane microfiltration was operated under the fixed mode with 8 min/2 min on/off driven by peristaltic pumps. To investigate the accumulation of aluminum salt in the microorganisms and its potential impacts on the treatment performance, the SRTs of two A/O-MBRs were controlled at 15 and 30 days, respectively. The whole experimental process was divided into two stages. Without dosing of PAC, the SRT-15d and SRT-30d reactors were performed to achieve stable performance for ammonium and COD removals at stage 1 (0–31 d). At stage 2 (32–55 d), 102.0 mg L−1 of PAC (28% Al2O3 (wt%), Guangfu Research Institute, Tianjin, China) which equaled to 20 mg Al L−1, was dosed into the SRT-15d and SRT-30d reactors to enhance phosphorus removal.
AOR =
NOR =
(mgNO2 N) CN = Cs t (gVSS d)
(3)
Where ΔCN represents the decrease of nitrite concentration (NO2−-N) and Cs is the sludge concentration in the conical flasks. Δt is the interval between determinations of NO2−-N concentration. 2.3.3. Nitrate utilization rate (NUR) tests The NUR tests were carried out in 500 mL sealed beakers. The solution was stripped using nitrogen gas for 10 min. Sodium nitrate (30 mg NO3−-N L−1) and sodium acetate (180 mg COD L−1) were dosed into the sealed beakers. The liquid samples were collected every 30 min and filtered with 0.45 μm filters for analyzing the concentrations of NO2−-N and NO3−-N. The NUR was calculated with the decreasing slope of NOx−-N (NO2−-N + NO3−-N) concentration with the time divided by the VSS concentration.
The inoculum was the activated sludge which was obtained from the sequencing batch reactor (SBR) in Zhuzhuanjing municipal treatment plant (Hefei city, China). The inoculated mixed liquid suspended solids (MLSS) and mixed liquid volatile suspended solids (MLVSS) concentrations of SRT-15d and SRT-30d reactors were 4.15 and 2.81 g L−1, respectively. The concentrations of ammonium (NH4+-N), phosphorus, and chemical oxygen demand (COD) were 40, 8, and 400 mg L−1, respectively in the synthetic wastewater. Besides, the feed nutrients contained (in mg L−1): KI 0.18, CuSO4·5H2O 0.003, CoCl·6H2O 0.15, H3BO3 0.15, FeCl3·6H2O 1.5, ZnSO4·7H2O 0.12, MnCl2·4H2O 0.12, Na2MoO4·2H2O 0.06.
2.3.4. Phosphorus release rate (PRR) and phosphorus uptake rate (PUR) tests The PRR and PUR tests were both carried out in 500 mL sealed beakers. Before the tests, the sludge was continuously aerated with sodium phosphate solution to make the phosphate accumulating organisms (PAOs) to uptake phosphorus adequately. Subsequently, dosing of sodium acetate (120 mg COD L−1 in sealed beakers) and stripping oxygen using nitrogen gas for 10 min were conducted for the PRR tests. The PUR tests were carried out following the PRR tests. At the beginning of PUR tests, the sodium phosphate solution were dosed into beakers (6 mg P L−1 in sealed beakers) with continuously aerated. The liquid samples were collected every 30 min and filtered with 0.45 μm filters to detect the concentration of phosphorus in both PPR and PUR tests. The PRR and PUR was employed to characterize the initial rate of the activity tests.
2.3. The bacterial activity tests of sludge 2.3.1. Specific oxygen uptake rate (SOUR) tests The SOUR was determined by the total rate of ammonia, nitrite, and carbon substrate oxidation with the biological and chemical processes, and biological endogenous respiration (Hass, 1979). The SOUR tests were carried out in 500 mL conical flasks with 2 g VSS L−1 of sludge. Besides, the concentrations of COD, NH4+-N, and NO2−-N in the conical flask were controlled at 200, 15, and 15 mg L−1, respectively. The concentration of dissolved oxygen (DO) was continuously recorded with a DO meter (HQ30d, HACH, America). The SOUR values were calculated as:
(mgO2 L 1) CDO = Cs t (gVSS min )
(2)
Where ΔCA is decreasing ammonia (NH4+-N) concentration and Cs is the concentration of sludge in the conical flasks. Δt is the interval between measurements of NH4+-N concentration.
2.2. Inoculum and wastewater characteristics
SOUR =
(mgNH4+ N ) CA = Cs t (gVSS d)
2.4. Analytical methods The concentrations of COD, NH4+-N, NO2−-N, NO3−-N, total phosphorus (TP), MLSS, and MLVSS were determined according to the Standard Method (APHA, 2012). The concentration of total nitrogen (TN) was defined as the combination of NH4+-N, NO2−-N, and NO3−N. The concentration of DO was measured with the DO meter (HQ30d, HACH, America). The concentrtion of Al in the effluent was determined by the chrome azurol S spectrophotometric method (GB/ T5750.6–2006) and the details of the process was referred to previous literature (Han et al., 2018). The atomic percentage of Al element on the surface of sludge was detected by an energy dispersive spectrometer (EDS) (FESEM, SU8020, Hitachi Co., Japan). The transmembrane press (TMP) was continuously determined online using a pressure sensors.
(1)
Where ΔCDO is the amount of DO concentration decreased and Cs is the concentration of sludge in the conical flasks. Δt represents the interval between measurements of oxygen concentration. 2.3.2. Ammonia oxidation rate (AOR) and nitrite oxidation rate (NOR) tests The AOR and NOR tests were referred to previous methods (Zubrowska-Sudol and Walczak, 2014). The AOR tests were conducted in 500 mL sealed beakers. The inoculum sludge concentration was 2 g VSS L−1 and the concentration of DO was controlled at around 2 mg O2
2.5. Analysis of mass balance 2.5.1. Biomass balance The biomass balance was used in this study to evaluate sludge 2
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production with organics consumption, as shown in Eq. (4) (Fan and Jin, 2009).
X V = Yobs Q (S0
3. Results and discussion 3.1. Influence of accumulated aluminum on SOUR of sludge
(4)
Se)
Fig. 1(a) shows the influence of accumulated aluminum on SOUR of sludge in the A/O-MBR. With the increasing amount of PAC was dosed into the sludge from 10 to 50 mg Al L−1, the inhibition degree of SOUR increased to 60.7% for 1.5 g VSS L−1 and 38.7% for 3.0 g VSS L−1. Interestingly, the higher concentration of sludge was slightly affected by accumulated aluminum. In a high MLVSS (MLVSS = 4.59 ± 0.31 g L−1) reactor, it was significant that the inhibition effect of accumulated salt on sludge was reduced, both in terms of microorganism growth and bacterial properties (Corsino et al., 2019). It indicated that the salt inhibition was directly related to the biomass in reactor. Liu et al. (2011) found that during the long-term reactor operation, the SOUR was significantly decreased after dosing of aluminum salt (11.2 mg L−1) at a long SRT without discharging sludge. As presented in Fig. 1(b), the inhibition degree of SOUR had a linear relationship with the accumulation of aluminum in sludge, as shown in Eq. (8).
Where S0 and Se are the substrate concentration in the influent and effluent, respectively in the A/O-MBR system. Q represents the amount of the wastewater treated by the reactor. ΔX represents the increased concentration of sludge in the reactor and the V is the volume of the reactor. Yobs represents the observed sludge yield which is the amount of sludge produced per the amount of substrate removed. The Yobs is used to evaluate the relationship of sludge growth and substrate consumption, and it can be defined as the ratio of cumulative generated sludge (P) to cumulative consumed substrate (C) (Chon et al., 2011). The substrate was characterized by organics as COD (Rodriguez Boluarte et al., 2016). Besides, we determined the VSS concentration at the beginning and the end of each operational period. Thus, the Yobs was referred to previous study (Sodhi et al., 2018) and can be written as flows:
Yobs =
(X end Xbeg ) Vw + Xd Vd t (gVSS ) P = = C (Qinf Cinf Qeff Ceff ) t (gCOD)
(5)
SOURinh = kinb c (Al) + b kinb = 2.10969, b= 10.59675,
Where the Xend stands for the sludge concentration at the end of each operational period while Xbeg stands for its concentration at the be¯ d is the average sludge concentration for each period. Vw is ginning. X the total working volume of the A/O-MBR system, and Vd is the volume of sludge discharged daily. Δt represents the duration of each period. Besides, Qinf and Qeff are the flow rate of the influent and effluent. Cinf and Ceff are represented as the COD concentration of the influent and effluent.
r 2 = 0.96347
(8)
Where SOURinb is the inhibition rate of SOUR (%) and the kinb is defined as the inhibition coefficient. The c(Al) is the concentration of aluminum salt in the sludge and the b is the constant. The result indicated that the kinb remained quite stable even with the increasing concentration of accumulated aluminum salt in sludge. The SOUR was closely related to the microbial metabolism of aerobic bacteria, such as AOB, NOB, and PAOs (Ma et al., 2019; Wang et al., 2016). During the process of nitrification, the decrease of nitrifying activity of sludge caused by aluminum salt would show a sharp
2.5.2. Aluminum balance The aluminum balance was to evaluate the accumulation of aluminum salt in the A/O-MBR. Eq. (6) is used to express the aluminum balance. (6)
Cd Q inf = Ca Vw + Ce Vd + Cf Qeff
Where the Cd is the dosing concentration of aluminum salt (20 mg Al L−1). Ca, Ce, and Cf are the concentration of aluminum salt in the reactor, the discharged sludge, and the effluent. Because no aluminum salt was detected in the effluent, we assumed that the aluminum salt accumulated and evenly distributed in the sludge. Thus, when we discharged the excess sludge, the excreted aluminum was regarded as a positive relationship to the amount of sludge discharged. With this assumption, the accumulated aluminum concentration (Cn) was described as the following equation.
Cn =
mAl ms
=
Cn 1 Cs (VW Vd ) + Cd Q inf (mgAl) = (gVSS ) , VW Cs
C0 = 0mgAl/ gVSS
(n
1) (7)
Where mAl is the total amount of aluminum and ms is the amount of sludge in the system. n represents the days after dosing of PAC while C0 represents the concentration of aluminum in the reactor the day before dosing of PAC. Cs is the concentration of sludge in the reactor. 2.6. Statistical analysis The method Analysis of Variance (ANOVA) was used for significance analysis of the experimental data (the performance of A/OMBR, the activity of AOB and NOB) at 0.05 level by using Microsoft Office Excel 2013 (Microsoft Corporation, USA) to determine p-value (p = probability), and p-values < 0.05 were considered significant (Ruhyadi et al., 2019).
Fig. 1. Influence of accumulated aluminum on SOUR of sludge in the A/OMBR. (a) SOUR, (b) Inhibition ratios of SOUR. 3
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Table 1 Treatment performance of A/O-MBRs at two different SRTs (SRT-15d-1: stage1, SRT = 15 d and without dosing of PAC. SRT-15d-2: stage2, SRT = 15 d and with dosing 102.0 mg L−1 of PAC. SRT-30d-1: stage1, SRT = 30 d and without dosing of PAC. SRT-30d-2: stage2, SRT = 30 d and with dosing 102.0 mg L−1 of PAC).
Influent Effluent
SRT-15d-1 SRT-30d-1 SRT-15d-2 SRT-30d-2
TN (mg L−1)
NH4+-N (mg L−1)
TP (mg L−1)
COD (mg L−1)
40.00 6.37 ± 7.35 ± 5.27 ± 6.49 ±
40.00 0.61 ± 0.76 ± 1.17 ± 1.61 ±
8.00 6.57 7.28 0.13 0.23
400.00 20.86 ± 24.67 ± 13.52 ± 15.43 ±
1.99 1.81 0.79 1.01
0.13 0.18 0.14 0.28
± ± ± ±
2.41 2.05 0.07 0.18
6.21 5.43 4.38 5.75
decrease of SOUR (Liu et al., 2011). With dosing of 40 mg L−1 PAC, the SOUR was inhibited and the removal efficiency of NH4+-N in long-term sequencing batch reactor (SBR) without discharging sludge was limited by the accumulated aluminum salt (Ji et al., 2015). Meanwile, the accumulation of aluminum salt in the sludge (from 0 to 30 mg Al g−1 TSS) showed an obvious inhibition on the microbial degradation (Chen et al., 2018), which also meant the inhibitory effect was correlated with the biomass. Therefore, controlling the accumulated aluminum in sludge might be an effective way to alleviate the inhibition effect on the microorganism. 3.2. Influence of accumulated aluminum on the treatment performance of A/O-MBRs at two different SRTs Table 1 shows the treatment performance of A/O-MBR reactor at two different SRTs. At the stage 1, the removal efficiencies of TN, NH4+-N, TP, and COD were 84.1%, 98.5%, 17.9%, and 94.8%, respectively at the SRT-15 reactor, while 81.6%, 98.1%, 9.0%, and 93.8%, respectively at the SRT-30 reactor (p = 0.88, 0.07, 0.73 and 0.19, respectively, compared to the removal efficiencies of TN, NH4+N, TP, and COD in the SRT-15d reactor at the stage 1). It was obvious that the SRT-15d reactor presented a better removal of ammonia and organic pollutants. As for different SRTs, Friedrich et al. (2015) reported the sludge at a shorter SRT would possess the maximum specific growth rate and endogenous respiration rate to show a higher bacterial activity and treatment performance. At the stage 2, the removal efficiencies of TN, NH4+-N, TP, and COD were 86.8%, 97.1%, 98.4%, and 96.6%, respectively in the SRT-15 reactor, and 83.8%, 96.0%, 97.1%, and 96.1%, respectively in the SRT-30 reactor (p = 0.02, 0.05, 0.38 and 0.41, respectively, p values are significant compared to the removal efficiencies of TN and NH4+-N in the SRT-15d reactor at the stage 2). Correspondingly, the removals of nitrogen, phosphorus, and COD in the SRT-15d and SRT-30d reactors changed obviously with dosing of aluminum salt. At the stage 2, although phosphorus and COD removals were enhanced with the dosing of aluminum salt, the effluent concentration of NH4+-N was obviously increased. Interestingly, the increase of NH4+-N concentration in effluent in the SRT-15d reactor (91.80%) was lower than that of the SRT-30d reactor (111.84%) which meant the inhibition of nitrification in the SRT-15d reactor was lower than the SRT-30d reactor. On the one hand, the shorter SRT could induce the sludge growth rate with better bacterial activity. Xu et al. (2015) pointed out that the bacterial growth rate was obviously improved with the increase of high organic loading at the shorter SRT. The MBR with a shorter SRT (SRT = 15d) led to higher EPS (Yu and Su, 2012). On the other hand, the shorter SRT might cause the change of sludge growth due to the decline of accumulated aluminum in the sludge. Because more sludge was discharged at a shorter SRT, more aluminum salt was also excreted (He et al., 2016). However, the quantitative relationship between the accumulated aluminum salts and the bacterial activity was still lacking.
Fig. 2. Influence of the accumulated aluminum on the observed sludge yield at different SRTs (SRT15-1: stage1, SRT = 15 d and without dosing of aluminum. SRT15-2: stage2, SRT = 15 d and with dosing of 20 mg Al L−1. SRT30-1: stage1, SRT = 30 d and without dosing of aluminum. SRT30-2: stage2, SRT = 30 d and with dosing of 20 mg Al L−1).
MLSS and MLVSS in SRT-15d reactor were increased from 2.36 g SS L−1 and 1.53 g VSS L−1 at stage 1–3.32 g SS L−1 and 2.08 g VSS L−1 at stage 2. Meanwhile, those in SRT-30d reactor were correspondingly increased from 2.62 g SS L−1 and 1.61 g VSS L−1 to 3.90 g SS L−1 and 2.40 g VSS L−1. At the stage 1, the observed sludge yields were 0.203 ± 0.010 and 0.156 ± 0.006 g VSS g−1 COD at the SRT-15d and SRT-30d reactors, respectively. After dosing of aluminum salt, the observed sludge yields correspondingly decreased to 0.185 ± 0.042 and 0.119 ± 0.051 g VSS g−1 COD, respectively. The short SRT could enhance the microorganisms renew and uptake organic compounds to synthesize new bacterial cell (Corsino et al., 2019). Furthermore, the accumulated aluminum suppressed the growth yield of sludge at the stage 2 and the decline of sludge yield at the SRT-15d reactor (8.9%) was lower than that of the SRT-30d reactor (23.7%). As shown in Fig. 3(a), it reflected the accumulation of aluminum salt in sludge of the SRT-15d and SRT-30d reactors. By theoretical calculation, the amount of accumulated aluminum in the SRT-15d and SRT-30d reactors were 130.87 and 156.94 mg Al g−1 SS, respectively, at the end of stage 2. With the mechanisms of charge neutralization, polymer bridging and electrostatic patch (Chai et al., 2014), most aluminum salts could accumulate in sludge and accumulated aluminum might bring negative effects on bacteria. As shown in Fig. 3(b), by calculation, the amount of accumulated aluminum in the SRT-15d reactor (230.59 mg Al g−1 VSS) was lower than that of the SRT-30d reactor (269.92 mg Al g−1 VSS) at the end of stage 2. It could be expected that the equilibrium accumulative concentration of aluminum salt would be achieved sooner at the SRT-15d reactor. As calculated, the equilibrium accumulative concentration of aluminum would be achieved on the 64th day (after dosing aluminum salt) in the SRT-15d reactor, while the SRT-30d reactor would reach the equilibrium value on the 107th day (after dosing aluminum salt). The expected
3.3. Analysis of mass balance at different SRTs The influence of the accumulated aluminum on the observed sludge yield at different SRTs is shown in Fig. 2. The average concentrations of 4
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Fig. 4. Influence of accumulated aluminum on the activity of AOB and NOB at different SRTs (SRT15-1: stage1, SRT = 15 d and without dosing of aluminum. SRT15-2: stage2, SRT = 15 d and with dosing of 20 mg Al L−1. SRT30-1: stage1, SRT = 30 d and without dosing of aluminum. SRT30-2: stage2, SRT = 30 d and with dosing of 20 mg Al L−1).
significant inhibition of aluminum salt on nitrification. Chen et al. (2019) reported that the high concentration of dosing metal ions (5 mg Cu2+ L−1 and 60 mg Fe3+ L−1) would weaken enzyme activities. Furthermore, because the ammonia monooxygenase of AOB affected more easily, the AOB was more suppressed severely than NOB due to the accumulated aluminum (Yang et al., 2019). The shorter SRT was prone to weaken the inhibitory effect of accumulated aluminum salt on the AOB and NOB. Liu et al. (2011) showed the inhibition of accumulated aluminum salt was more serious on the AOB with the long-term dosing of 56 mg Al L−1, and the nitrification could even decline to 20% without discharging aluminum salt. With increased metal salts dosed into sludge, the ammonia oxidation rates showed a downward trend (Chen et al., 2019). These findings indicated that the higher concentration of aluminum salt in biomass would significantly enhance the suppressing effect on activity of AOB and NOB. Thus, the reduction of accumulated aluminum salt in sludge would be the key step to recover the biological activity. In this study, controlling SRT could effectively reduce the accumulation of aluminum in sludge, and the inhibition effect of aluminum on activity of AOB and NOB was weakened at shorter SRT (in SRT-15d reactor). Therefore, controlling shorter SRT could be employed to alleviate the suppressing effect on nitrification by accumulated aluminum salt in the A/O-MBR.
Fig. 3. Effect of different SRTs on accumulation of aluminum in the sludge. (a) Aluminum salt in MLSS of sludge, (b) Aluminum salt in MLVSS of sludge.
equilibrium concentrations of accumulated aluminum in the sludge were 289.2 and 489.6 mg Al g−1 VSS in the SRT-15d and SRT-30d reactors, respectively. He et al. (2016) showed that the sludge structure and properties would become worse under high salinity environment (> 1 wt% salinity). Besides, it was confirmed that high concentration of aluminum salt covering on sludge would compress the sludge EPS (Cao et al., 2016), which also influenced the bacterial activities with the increasing accmulated aluminum. The high concentration of metal salt would exceed the cell surface capacity (related to the EPS capacity), even entered into the cells to cause the death of bacteria (Ma et al., 2018). Due to the higher growth yield of sludge and lower rate of aluminum accumulation in the shorter SRT (SRT = 15 d) reactor, the accumulated aluminum salt in the sludge would be controlled at a relatively low level. Therefore, controlling shorter SRT could be used to alleviate inhibition of nitrification by accumulated aluminum salt in the A/O-MBR, but the expected result required balancing the sludge growth rate with the aluminum accumulation rate.
3.5. Influence of accumulated aluminum on the activity of biological phosphorus removal and denitrification of A/O-MBRs at two different SRTs The effect of accumulation of aluminum salt on denitrification activity of sludge is shown in Fig. S2. Without dosing of PAC, the NOx−-N removal rates were 67.04% and 26.20%, respectively for the SRT-15d and SRT-30d reactors at 30 min. With accumulation of aluminum, the NOx−-N removals obviously increased to 82.31% and 42.19%, respectively in the SRT-15d and SRT-30d reactors. After dosing of aluminum salt, the denitrification activity of sludge in the both reactors were improved. The improvement of sludge denitrification activity in a longer SRT (SRT = 30 d) was more obvious which was caused by the denser flocculating structure to provide anaerobic environment in MBRs with a higher concentration of aluminum salt (Cao et al., 2016). As reported the metal ions were also the important factor to the specific reductase biosynthesis of sludge denitrification activity (Zhu et al., 2013). Although it was still no clear why NUR was improved with accumulation of aluminum salt, the denitrification activity of sludge at
3.4. Influence of accumulated aluminum on the activity of AOB and NOB in A/O-MBRs at two different SRTs The influence of accumulated aluminum on the activity of AOB and NOB at different SRTs is shown in Fig. 4. With the accumulation of aluminum salt in the reactor, the activity of AOB was decreased by 4.4% and 10.9% at the SRT-15d and SRT-30d reactors respectively (p = 0.27 and 0.04, respectively, p value is significant compared to the activity of AOB after aluminum accumulation in the SRT-30d reactor), while the activity of NOB was decreased by 18.3% and 23.9%, respectively (p = 0.07 and 5.18 × 10−4, respectively, p values are significant compared to the activity of NOB after aluminum accumulation in the SRT-15d and SRT-30d reactor). After dosing of PAC, it indicated a 5
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the shorter SRT (SRT = 15 d) was still much higher than that at the longer SRT (SRT = 30 d). Thus, controlling shorter SRTs would be beneficial to denitrification with the accumulation of aluminum salt. The dosing of PAC could chemically reduce phosphorus concentration in effluent, whereas it also influenced biological phosphorus removal. The profile of phosphorus release and uptake with and without accumulation of aluminum at different SRTs is shown as Fig. S3. Before dosing of PAC, the phosphorus release concentrations were 14.15 and 17.50 mg L−1, respectively in the SRT-15d and SRT-30d reactors at 150 min. After dosing of aluminum salt, they decreased to 11.72 and 13.60 mg L−1, respectively in the SRT-15d and SRT-30d reactors. It indicated that the sludge with accumulated aluminum salt showed a suppressing effect on phosphorus removal. Liu et al. (2016) described the phosphorus adoption in aluminum-sludge and it might refer to the role of neutralization and bio-chemical bridging (Cui et al., 2018). The incomplete phosphorus release also occurred in our experiment which could be explained as the adsorption of aluminum salt. Still, controlling the shorter SRT (SRT = 15 d) could weaken the suppressing effect of phosphorus release. Also, in the process of phosphorus uptake, it showed that before dosing of PAC, the phosphorus uptake rate were 20.80% and 29.93%, respectively for the SRT-15d and SRT30d reactors at 270 min. However, after accumulation of aluminum salt, the phosphorus uptake rates were 95.80% and 94.43%, respectively in the SRT-15d and SRT-30d reactors. It reflected that the accumulated aluminum salt also had a suppressing effect on phosphorus uptake. Although the biological activity of phosphorus removal was suppressed by accumulated salt, the A/O-MBR with dosing of aluminum salt showed an excellent performance on chemical phosphorus removal.
inappropriately influence our work, there is no commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was funded by the National Natural Science Foundation of China (51878232) and Foundation of Anhui Shun Yu Water Co., Ltd. (W2018JSKF0137). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ibiod.2019.104755. References APHA, 2012. Standard methods for the examination of water and wastewater. Washington, D.C 21 st Edition. Cao, B., Zhang, W., Wang, Q., Huang, Y., Meng, C., Wang, D., 2016. Wastewater sludge dewaterability enhancement using hydroxyl aluminum conditioning: role of aluminum speciation. Water Res. 105, 615–624. Chai, S.L., Robinson, J., Mei, F.C., 2014. A review on application of flocculants in wastewater treatment. Process Saf. Environ. 92, 489–508. Chen, Y., Wang, Q., Zhao, S., Yang, W., Wang, H., Jia, W., 2019. Removal of nutrients and emission of nitrous oxide during simultaneous nitrification, denitrification and phosphorus removal process with metal ions addition. Int. Biodeterior. Biodegrad. 142, 143–150. Chen, Y., Wu, Y., Wang, D., Li, H., Wang, Q., Liu, Y., Peng, L., Yang, Q., Li, X., Zeng, G., Chen, Y., 2018. Understanding the mechanisms of how poly aluminium chloride inhibits short-chain fatty acids production from anaerobic fermentation of waste activated sludge. Chem. Eng. J. 334, 1351–1360. Chon, D.H., Rome, M., Kim, Y.M., Park, K.Y., Park, C., 2011. Investigation of the sludge reduction mechanism in the anaerobic side-stream reactor process using several control biological wastewater treatment processes. Water Res. 45, 6021–6029. Corsino, S.F., Capodici, M., Di Pippo, F., Tandoi, V., Torregrossa, M., 2019. Comparison between kinetics of autochthonous marine bacteria in activated sludge and granular sludge systems at different salinity and SRTs. Water Res. 148, 425–437. Cui, X., Huo, M., Chen, C., Yu, Z., Zhou, C., Li, A., Qiao, B., Zhou, D., Crittenden, J.C., 2018. Low concentrations of Al(III) accelerate the formation of biofilm: multiple effects of hormesis and flocculation. Sci. Total Environ. 634, 516–524. Falahti-Marvast, H., Karimi-Jashni, A., 2015. Performance of simultaneous organic and nutrient removal in a pilot scale anaerobic–anoxic–oxic membrane bioreactor system treating municipal wastewater with a high nutrient mass ratio. Int. Biodeterior. Biodegrad. 104, 363–370. Fan, J., Jin, Z., 2009. Water Quality Engineering, 1 st Edition. China Architecture & Building Press, Beijing, pp. 352–353. Friedrich, M., Takács, I., Tränckner, J., 2015. Physiological adaptation of growth kinetics in activated sludge. Water Res. 85, 22–30. Han, X., Zhou, Z., Mei, X., Ma, Y., Xie, Z., 2018. Influence of fermentation liquid from waste activated sludge on anoxic/oxic- membrane bioreactor performance: nitrogen removal, membrane fouling and microbial community. Bioresour. Technol. 250, 699–707. Hass, C.N., 1979. Oxygen uptake rate as an activated sludge control parameter. J. Water Pollut. Control Fed. 51, 938–943. He, H., Chen, Y., Xiang, L., Yan, C., Yang, C., Zeng, G., 2016. Influence of salinity on microorganisms in activated sludge processes: a review. Int. Biodeterior. Biodegrad. 119, 520–527. Ji, B., Kai, Y., Wang, H., 2015. Impacts of poly-aluminum chloride addition on activated sludge and the treatment efficiency of SBR. Desalin. Water Treat. 54, 2376–2381. Kochkodan, V., Hilal, N., 2015. A comprehensive review on surface modified polymer membranes for biofouling mitigation. Desalination 356, 187–207. Li, R.H., Wang, X.M., Li, X.Y., 2018. A membrane bioreactor with iron dosing and acidogenic co-fermentation for enhanced phosphorus removal and recovery in wastewater treatment. Water Res. 129, 402–412. Liu, R., Zhao, Y., Sibille, C., Ren, B., 2016. Evaluation of natural organic matter release from alum sludge reuse in wastewater treatment and its role in P adsorption. Chem. Eng. J. 302, 120–127. Liu, Y., Shi, H., Li, W., Hou, Y., He, M., 2011. Inhibition of chemical dose in biological phosphorus and nitrogen removal in simultaneous chemical precipitation for phosphorus removal. Bioresour. Technol. 102, 4008–4012. Londono, S.C., Hartnett, H.E., Williams, L.B., 2017. Antibacterial activity of aluminum in clay from the colombian amazon. Environ. Sci. Technol. 51, 2401–2408. Ma, B., Li, Z., Wang, S., Liu, Z., Li, S., She, Z., Yu, N., Zhao, C., Jin, C., Zhao, Y., Guo, L., Gao, M., 2019. Insights into the effect of nickel (Ni(II)) on the performance, microbial enzymatic activity and extracellular polymeric substances of activated sludge. Environ. Pollut. 251, 81–89. Ma, Y., Yuan, D., Mu, B., Zhou, J., Zhang, X., 2018. Reactor performance, biofilm property and microbial community of anaerobic ammonia-oxidizing bacteria under longterm exposure to elevated Cu (II). Int. Biodeterior. Biodegrad. 129, 156–162.
3.6. Practical implications In this study, with dosing of PAC, the increasing rate of TMP and the effluent concentration of phosphorus obviously decreased with the dosing of PAC which indicated that it could enhance phosphorus removal and effectively reduce membrane fouling (Fig. S3 and Fig. S4). Also, the denitrification rates of the SRT-15d and SRT-30d reactors were improved with the accumulation of aluminum. The denitrification activity of sludge at the shorter SRT (SRT = 15 d) was also much higher than that of the longer SRT (SRT = 30 d). Although the accumulated aluminum salts would inhibit the activity of AOB and NOB, the dosing of PAC could bring some benefits on the performance of A/O-MBRs. Controlling shorter SRT could alleviate inhibition of nitration and nitrosation in the A/O-MBRs. At the same time, some researchers had developed new pathways for recycling aluminum-containing sludge (Liu et al., 2016), and it was more potential to be reused as an emerged substrates in constructed wetland for enhancing phosphorus removal (Yang et al., 2018). Therefore, controlling SRT can be used as an important tool to improve the performance of A/O-MBRs with dosing of PAC. 4. Conclusion The main conclusions could be drawn as follows: (1) The inhibition rate of aluminum salt on SOUR was positively correlated with the content of aluminum salt in sludge. (2) The accumulated amount and rate of aluminum salt of sludge at the shorter SRT was lower than that of the longer SRT. (3) Controlling shorter SRT could be employed as an effective tool to alleviate inhibition of nitrification by accumulated aluminum in A/ O-MBR. Conflicts of interest statement Our all the authors declare that we have no financial and personal relationships with other people or organizations that can 6
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C. Qiu, et al. Park, J., Yamashita, N., Tanaka, H., 2018. Membrane fouling control and enhanced removal of pharmaceuticals and personal care products by coagulation-MBR. Chemosphere 197, 467–476. Riaz, M., Yan, L., Wu, X., Hussain, S., Aziz, O., Jiang, C., 2018. Mechanisms of organic acids and boron induced tolerance of aluminum toxicity: a review. Ecotoxicol. Environ. Saf. 165, 25–35. Rodriguez Boluarte, I.A., Andersen, M., Pramanik, B.K., Chang, C.-Y., Bagshaw, S., Farago, L., Jegatheesan, V., Shu, L., 2016. Reuse of car wash wastewater by chemical coagulation and membrane bioreactor treatment processes. Int. Biodeterior. Biodegrad. 113, 44–48. Ruhyadi, R., Chen, Y., Shen, N., Yan, W., Liang, Z., Wang, H., Wang, G., 2019. Multiple uses of magnesium chloride during waste activated sludge alkaline fermentation. Bioresour. Technol. 290, 121792. She, Q., Wang, R., Fane, A.G., Tang, C.Y., 2016. Membrane fouling in osmotically driven membrane processes: a review. J. Membr. Sci. 499, 201–233. Sodhi, V., Bansal, A., Jha, M.K., 2018. Excess sludge disruption and pollutant removal from tannery effluent by upgraded activated sludge system. Bioresour. Technol. 263, 613–624. Wang, S., Gao, M., She, Z., Zheng, D., Jin, C., Guo, L., Zhao, Y., Li, Z., Wang, X., 2016. Long-term effects of ZnO nanoparticles on nitrogen and phosphorus removal, microbial activity and microbial community of a sequencing batch reactor. Bioresour. Technol. 216, 428–436. Xiao, K., Liang, S., Wang, X., Chen, C., Huang, X., 2019. Current state and challenges of full-scale membrane bioreactor applications: a critical review. Bioresour. Technol. 271, 473–481.
Xu, M., Li, P., Tang, T., Hu, Z., 2015. Roles of SRT and HRT of an algal membrane bioreactor system with a tanks-in-series configuration for secondary wastewater effluent polishing. Ecol. Eng. 85, 257–264. Yang, C.H., Qiu, C.G., He, C.H., Hu, Z.H., Wang, W., 2019. Influence of aluminium accumulation on biological nitrification and phosphorus removal in an anoxic–oxic membrane bioreactor. Environ. Sci. Pollut. Res. Yang, X., Song, H., Chen, M., Cheng, B., 2011. Characterizing membrane foulants in MBR with addition of polyferric chloride to enhance phosphorus removal. Bioresour. Technol. 102, 9490–9496. Yang, Y., Zhao, Y., Liu, R., Morgan, D., 2018. Global development of various emerged substrates utilized in constructed wetlands. Bioresour. Technol. 261, 441–452. Yu, T., Su, X., 2012. Relation between the stability of activated sludge flocs and membrane fouling in MBR: under different SRTs. Bioresour. Technol. 118, 477–482. Yu, Z., Song, Z., Wen, X., Xia, H., 2015. Using polyaluminum chloride and polyacrylamide to control membrane fouling in a cross-flow anaerobic membrane bioreactor. J. Membr. Sci. 479, 20–27. Zhang, Z., Wang, Y., Leslie, G.L., Waite, T.D., 2015. Effect of ferric and ferrous iron addition on phosphorus removal and fouling in submerged membrane bioreactors. Water Res. 69, 210–222. Zhu, X., Chen, Y., Chen, H., Li, X., Peng, Y., Wang, S., 2013. Minimizing nitrous oxide in biological nutrient removal from municipal wastewater by controlling copper ion concentrations. Appl. Microbiol. Biotechnol. 97, 1325–1334. Zubrowska-Sudol, M., Walczak, J., 2014. Effects of mechanical disintegration of activated sludge on the activity of nitrifying and denitrifying bacteria and phosphorus accumulating organisms. Water Res. 61, 200–209.
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Controlling sludge retention time to alleviate inhibition of nitrosation and nitration by accumulated aluminum in an A/O-MBR
T
Chenggang Qiu1, Chuanhe Yang1, Chunhua He, Miao Gong, Zhenhu Hu, Wei Wang* Department of Municipal Engineering, School of Civil Engineering, Hefei University of Technology, Hefei, 230009, China
ARTICLE INFO
ABSTRACT
Keywords: Sludge retention time Poly-aluminum chloride Anoxic-oxic membrane bioreactor Biological phosphorus removal Nitrification Inhibition
Severe membrane fouling and limited effect of biological phosphorus removal are the major drawbacks for anoxicoxic membrane bioreactor (A/O-MBR). The dosing of poly-aluminum chloride in A/O-MBR can reduce membrane fouling and improve phosphorus removal certainly, but it may bring potential impacts on the specific oxygen uptake rate of sludge and the activity of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria with accumulated aluminum. Controlling sludge retention time (SRT) was firstly investigated to evaluate the effect on alleviating inhibition caused by accumulated aluminum in A/O-MBR. As compared to the performance without dosing of poly-aluminum chloride, the effluent ammonia concentration in the SRT-15d reactor was increased by 91.80% lower than that of SRT-30d reactor (111.84%). Due to higher sludge yield rate in the SRT-15d reactor, the amount of accumulated aluminum was 39.33 mg Al g−1 VSS less than that of the SRT-30d reactor. Besides, the decline of the activity of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the SRT-15d reactor was 4.4% and 18.3%, respectively lower than that of the SRT-30d reactor. Therefore, controlling shorter SRT can be used as an effective tool to alleviate inhibition of nitration and nitrosation by accumulated aluminum in A/O-MBR.
1. Introduction Anoxic-oxic membrane bioreactor (A/O-MBR) is a promising technology for wastewater treatment due to its advantages of reduced footprint, low sludge yield, and high quality of effluent (She et al., 2016). However, the membrane fouling is an inevitable problem (Kochkodan and Hilal, 2015). Meanwhile, with complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), the A/ O-MBR shows a weakness of biological phosphorus removal in a long SRT operation (Falahti-Marvast and Karimi-Jashni, 2015; Xiao et al., 2019). In general, some chemical coagulants such as polyferric chloride (Li et al., 2018; Yang et al., 2011) and polyaluminium chloride (Yu et al., 2015) were employed to simultaneously alleviate membrane fouling and enhance phosphorus removal. Nevertheless, it might bring potential impact on the microorganisms by the accumulated metal salts in the A/O-MBR. Previous studies aimed at the effect of chemical coagulants on reducing membrane fouling and achieving excellent phosphorus removal, ignoring the accumulation of metal salts, especially for aluminum salts in the A/O-MBR with a long SRT (Park et al., 2018; Zhang et al., 2015). Until recent years, it was reported that the trivalent cations such like Al3+ could accelerate lipid oxidation on the bacterial membrane
(Londono et al., 2017) and affect the bacterial activity of sludge such as ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) (Liu et al., 2011). The polymeric aluminum salt covering on the sludge would lead to both changes of sludge morphology and properties to inhibit the microbial metabolism (Cao et al., 2016). Even worse, the toxicity of aluminum salts to cells was expressed as damaging cell wall, interrupting the transport of molecules across cell membranes and impairing intercellular process (Riaz et al., 2018). The inhibition of accumulated aluminum salts on the microorganisms was more easily ignored in wastewater treatment. In fact, Liu et al. (2011) reviewed that the accumulated aluminum salt had an inhibitory effect on the bacterial activity with long-term dosing of aluminum salt. Despite the phenomena of PAC inhibition on the microorganisms have been found in the past, the accumulation rate and control method of aluminum in sludge has not been documented before. The aims of this study were to investigate the relationship between the accumulated aluminum salt and the inhibition of specific oxygen uptake rate of sludge. Furthermore, the performance of A/O-MBR without and with dosing of PAC was compared at different SRTs. Finally, the mechanism of controlling SRT to alleviate the inhibition of accumulated aluminum in the A/O-MBR was discussed.
*
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Wang). 1 Co-first author. https://doi.org/10.1016/j.ibiod.2019.104755 Received 22 May 2019; Received in revised form 23 July 2019; Accepted 5 August 2019 0964-8305/ © 2019 Elsevier Ltd. All rights reserved.
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L−1. Ammonia chloride (30 mg NH4+-N L−1) was dosed into the sealed beakers. The liquid samples were collected and filtered with 0.45 μm filters to measure the concentration of NH4+-N. The AOR was calculated as formula (2). Similar to the AOR tests, the NOR tests were conducted by dosing of sodium nitrite (30 mg NO2−-N L−1) in the sealed beakers and measuring the decreasing slope of NO2−-N concentration as formula (3).
2. Materials and methods 2.1. Experimental setup and operating conditions The reactor was made up of an A/O reactor and a submerged hollow fiber polyvinylidene fluoride (PVDF) microfiltration membrane module with the 0.2 μm of pore size and 15 L m−2 h−1 of membrane flux (as shown in Fig. S1). The reactor with a 6.1 L of working volume was operated at a 12 h of HRT and room temperature. Two pumps were employed to circulate wastewater and sludge. Before dosing PAC, the pH in the effluent was at the range of 7.4–8.4, while it slightly decreased to 7.3–8.1 after dosing of PAC. Besides, the concentration of DO in the aerobic zone was controlled at the range of 2–4 mg O2 L−1 during the whole operation. The membrane microfiltration was operated under the fixed mode with 8 min/2 min on/off driven by peristaltic pumps. To investigate the accumulation of aluminum salt in the microorganisms and its potential impacts on the treatment performance, the SRTs of two A/O-MBRs were controlled at 15 and 30 days, respectively. The whole experimental process was divided into two stages. Without dosing of PAC, the SRT-15d and SRT-30d reactors were performed to achieve stable performance for ammonium and COD removals at stage 1 (0–31 d). At stage 2 (32–55 d), 102.0 mg L−1 of PAC (28% Al2O3 (wt%), Guangfu Research Institute, Tianjin, China) which equaled to 20 mg Al L−1, was dosed into the SRT-15d and SRT-30d reactors to enhance phosphorus removal.
AOR =
NOR =
(mgNO2 N) CN = Cs t (gVSS d)
(3)
Where ΔCN represents the decrease of nitrite concentration (NO2−-N) and Cs is the sludge concentration in the conical flasks. Δt is the interval between determinations of NO2−-N concentration. 2.3.3. Nitrate utilization rate (NUR) tests The NUR tests were carried out in 500 mL sealed beakers. The solution was stripped using nitrogen gas for 10 min. Sodium nitrate (30 mg NO3−-N L−1) and sodium acetate (180 mg COD L−1) were dosed into the sealed beakers. The liquid samples were collected every 30 min and filtered with 0.45 μm filters for analyzing the concentrations of NO2−-N and NO3−-N. The NUR was calculated with the decreasing slope of NOx−-N (NO2−-N + NO3−-N) concentration with the time divided by the VSS concentration.
The inoculum was the activated sludge which was obtained from the sequencing batch reactor (SBR) in Zhuzhuanjing municipal treatment plant (Hefei city, China). The inoculated mixed liquid suspended solids (MLSS) and mixed liquid volatile suspended solids (MLVSS) concentrations of SRT-15d and SRT-30d reactors were 4.15 and 2.81 g L−1, respectively. The concentrations of ammonium (NH4+-N), phosphorus, and chemical oxygen demand (COD) were 40, 8, and 400 mg L−1, respectively in the synthetic wastewater. Besides, the feed nutrients contained (in mg L−1): KI 0.18, CuSO4·5H2O 0.003, CoCl·6H2O 0.15, H3BO3 0.15, FeCl3·6H2O 1.5, ZnSO4·7H2O 0.12, MnCl2·4H2O 0.12, Na2MoO4·2H2O 0.06.
2.3.4. Phosphorus release rate (PRR) and phosphorus uptake rate (PUR) tests The PRR and PUR tests were both carried out in 500 mL sealed beakers. Before the tests, the sludge was continuously aerated with sodium phosphate solution to make the phosphate accumulating organisms (PAOs) to uptake phosphorus adequately. Subsequently, dosing of sodium acetate (120 mg COD L−1 in sealed beakers) and stripping oxygen using nitrogen gas for 10 min were conducted for the PRR tests. The PUR tests were carried out following the PRR tests. At the beginning of PUR tests, the sodium phosphate solution were dosed into beakers (6 mg P L−1 in sealed beakers) with continuously aerated. The liquid samples were collected every 30 min and filtered with 0.45 μm filters to detect the concentration of phosphorus in both PPR and PUR tests. The PRR and PUR was employed to characterize the initial rate of the activity tests.
2.3. The bacterial activity tests of sludge 2.3.1. Specific oxygen uptake rate (SOUR) tests The SOUR was determined by the total rate of ammonia, nitrite, and carbon substrate oxidation with the biological and chemical processes, and biological endogenous respiration (Hass, 1979). The SOUR tests were carried out in 500 mL conical flasks with 2 g VSS L−1 of sludge. Besides, the concentrations of COD, NH4+-N, and NO2−-N in the conical flask were controlled at 200, 15, and 15 mg L−1, respectively. The concentration of dissolved oxygen (DO) was continuously recorded with a DO meter (HQ30d, HACH, America). The SOUR values were calculated as:
(mgO2 L 1) CDO = Cs t (gVSS min )
(2)
Where ΔCA is decreasing ammonia (NH4+-N) concentration and Cs is the concentration of sludge in the conical flasks. Δt is the interval between measurements of NH4+-N concentration.
2.2. Inoculum and wastewater characteristics
SOUR =
(mgNH4+ N ) CA = Cs t (gVSS d)
2.4. Analytical methods The concentrations of COD, NH4+-N, NO2−-N, NO3−-N, total phosphorus (TP), MLSS, and MLVSS were determined according to the Standard Method (APHA, 2012). The concentration of total nitrogen (TN) was defined as the combination of NH4+-N, NO2−-N, and NO3−N. The concentration of DO was measured with the DO meter (HQ30d, HACH, America). The concentrtion of Al in the effluent was determined by the chrome azurol S spectrophotometric method (GB/ T5750.6–2006) and the details of the process was referred to previous literature (Han et al., 2018). The atomic percentage of Al element on the surface of sludge was detected by an energy dispersive spectrometer (EDS) (FESEM, SU8020, Hitachi Co., Japan). The transmembrane press (TMP) was continuously determined online using a pressure sensors.
(1)
Where ΔCDO is the amount of DO concentration decreased and Cs is the concentration of sludge in the conical flasks. Δt represents the interval between measurements of oxygen concentration. 2.3.2. Ammonia oxidation rate (AOR) and nitrite oxidation rate (NOR) tests The AOR and NOR tests were referred to previous methods (Zubrowska-Sudol and Walczak, 2014). The AOR tests were conducted in 500 mL sealed beakers. The inoculum sludge concentration was 2 g VSS L−1 and the concentration of DO was controlled at around 2 mg O2
2.5. Analysis of mass balance 2.5.1. Biomass balance The biomass balance was used in this study to evaluate sludge 2
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production with organics consumption, as shown in Eq. (4) (Fan and Jin, 2009).
X V = Yobs Q (S0
3. Results and discussion 3.1. Influence of accumulated aluminum on SOUR of sludge
(4)
Se)
Fig. 1(a) shows the influence of accumulated aluminum on SOUR of sludge in the A/O-MBR. With the increasing amount of PAC was dosed into the sludge from 10 to 50 mg Al L−1, the inhibition degree of SOUR increased to 60.7% for 1.5 g VSS L−1 and 38.7% for 3.0 g VSS L−1. Interestingly, the higher concentration of sludge was slightly affected by accumulated aluminum. In a high MLVSS (MLVSS = 4.59 ± 0.31 g L−1) reactor, it was significant that the inhibition effect of accumulated salt on sludge was reduced, both in terms of microorganism growth and bacterial properties (Corsino et al., 2019). It indicated that the salt inhibition was directly related to the biomass in reactor. Liu et al. (2011) found that during the long-term reactor operation, the SOUR was significantly decreased after dosing of aluminum salt (11.2 mg L−1) at a long SRT without discharging sludge. As presented in Fig. 1(b), the inhibition degree of SOUR had a linear relationship with the accumulation of aluminum in sludge, as shown in Eq. (8).
Where S0 and Se are the substrate concentration in the influent and effluent, respectively in the A/O-MBR system. Q represents the amount of the wastewater treated by the reactor. ΔX represents the increased concentration of sludge in the reactor and the V is the volume of the reactor. Yobs represents the observed sludge yield which is the amount of sludge produced per the amount of substrate removed. The Yobs is used to evaluate the relationship of sludge growth and substrate consumption, and it can be defined as the ratio of cumulative generated sludge (P) to cumulative consumed substrate (C) (Chon et al., 2011). The substrate was characterized by organics as COD (Rodriguez Boluarte et al., 2016). Besides, we determined the VSS concentration at the beginning and the end of each operational period. Thus, the Yobs was referred to previous study (Sodhi et al., 2018) and can be written as flows:
Yobs =
(X end Xbeg ) Vw + Xd Vd t (gVSS ) P = = C (Qinf Cinf Qeff Ceff ) t (gCOD)
(5)
SOURinh = kinb c (Al) + b kinb = 2.10969, b= 10.59675,
Where the Xend stands for the sludge concentration at the end of each operational period while Xbeg stands for its concentration at the be¯ d is the average sludge concentration for each period. Vw is ginning. X the total working volume of the A/O-MBR system, and Vd is the volume of sludge discharged daily. Δt represents the duration of each period. Besides, Qinf and Qeff are the flow rate of the influent and effluent. Cinf and Ceff are represented as the COD concentration of the influent and effluent.
r 2 = 0.96347
(8)
Where SOURinb is the inhibition rate of SOUR (%) and the kinb is defined as the inhibition coefficient. The c(Al) is the concentration of aluminum salt in the sludge and the b is the constant. The result indicated that the kinb remained quite stable even with the increasing concentration of accumulated aluminum salt in sludge. The SOUR was closely related to the microbial metabolism of aerobic bacteria, such as AOB, NOB, and PAOs (Ma et al., 2019; Wang et al., 2016). During the process of nitrification, the decrease of nitrifying activity of sludge caused by aluminum salt would show a sharp
2.5.2. Aluminum balance The aluminum balance was to evaluate the accumulation of aluminum salt in the A/O-MBR. Eq. (6) is used to express the aluminum balance. (6)
Cd Q inf = Ca Vw + Ce Vd + Cf Qeff
Where the Cd is the dosing concentration of aluminum salt (20 mg Al L−1). Ca, Ce, and Cf are the concentration of aluminum salt in the reactor, the discharged sludge, and the effluent. Because no aluminum salt was detected in the effluent, we assumed that the aluminum salt accumulated and evenly distributed in the sludge. Thus, when we discharged the excess sludge, the excreted aluminum was regarded as a positive relationship to the amount of sludge discharged. With this assumption, the accumulated aluminum concentration (Cn) was described as the following equation.
Cn =
mAl ms
=
Cn 1 Cs (VW Vd ) + Cd Q inf (mgAl) = (gVSS ) , VW Cs
C0 = 0mgAl/ gVSS
(n
1) (7)
Where mAl is the total amount of aluminum and ms is the amount of sludge in the system. n represents the days after dosing of PAC while C0 represents the concentration of aluminum in the reactor the day before dosing of PAC. Cs is the concentration of sludge in the reactor. 2.6. Statistical analysis The method Analysis of Variance (ANOVA) was used for significance analysis of the experimental data (the performance of A/OMBR, the activity of AOB and NOB) at 0.05 level by using Microsoft Office Excel 2013 (Microsoft Corporation, USA) to determine p-value (p = probability), and p-values < 0.05 were considered significant (Ruhyadi et al., 2019).
Fig. 1. Influence of accumulated aluminum on SOUR of sludge in the A/OMBR. (a) SOUR, (b) Inhibition ratios of SOUR. 3
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Table 1 Treatment performance of A/O-MBRs at two different SRTs (SRT-15d-1: stage1, SRT = 15 d and without dosing of PAC. SRT-15d-2: stage2, SRT = 15 d and with dosing 102.0 mg L−1 of PAC. SRT-30d-1: stage1, SRT = 30 d and without dosing of PAC. SRT-30d-2: stage2, SRT = 30 d and with dosing 102.0 mg L−1 of PAC).
Influent Effluent
SRT-15d-1 SRT-30d-1 SRT-15d-2 SRT-30d-2
TN (mg L−1)
NH4+-N (mg L−1)
TP (mg L−1)
COD (mg L−1)
40.00 6.37 ± 7.35 ± 5.27 ± 6.49 ±
40.00 0.61 ± 0.76 ± 1.17 ± 1.61 ±
8.00 6.57 7.28 0.13 0.23
400.00 20.86 ± 24.67 ± 13.52 ± 15.43 ±
1.99 1.81 0.79 1.01
0.13 0.18 0.14 0.28
± ± ± ±
2.41 2.05 0.07 0.18
6.21 5.43 4.38 5.75
decrease of SOUR (Liu et al., 2011). With dosing of 40 mg L−1 PAC, the SOUR was inhibited and the removal efficiency of NH4+-N in long-term sequencing batch reactor (SBR) without discharging sludge was limited by the accumulated aluminum salt (Ji et al., 2015). Meanwile, the accumulation of aluminum salt in the sludge (from 0 to 30 mg Al g−1 TSS) showed an obvious inhibition on the microbial degradation (Chen et al., 2018), which also meant the inhibitory effect was correlated with the biomass. Therefore, controlling the accumulated aluminum in sludge might be an effective way to alleviate the inhibition effect on the microorganism. 3.2. Influence of accumulated aluminum on the treatment performance of A/O-MBRs at two different SRTs Table 1 shows the treatment performance of A/O-MBR reactor at two different SRTs. At the stage 1, the removal efficiencies of TN, NH4+-N, TP, and COD were 84.1%, 98.5%, 17.9%, and 94.8%, respectively at the SRT-15 reactor, while 81.6%, 98.1%, 9.0%, and 93.8%, respectively at the SRT-30 reactor (p = 0.88, 0.07, 0.73 and 0.19, respectively, compared to the removal efficiencies of TN, NH4+N, TP, and COD in the SRT-15d reactor at the stage 1). It was obvious that the SRT-15d reactor presented a better removal of ammonia and organic pollutants. As for different SRTs, Friedrich et al. (2015) reported the sludge at a shorter SRT would possess the maximum specific growth rate and endogenous respiration rate to show a higher bacterial activity and treatment performance. At the stage 2, the removal efficiencies of TN, NH4+-N, TP, and COD were 86.8%, 97.1%, 98.4%, and 96.6%, respectively in the SRT-15 reactor, and 83.8%, 96.0%, 97.1%, and 96.1%, respectively in the SRT-30 reactor (p = 0.02, 0.05, 0.38 and 0.41, respectively, p values are significant compared to the removal efficiencies of TN and NH4+-N in the SRT-15d reactor at the stage 2). Correspondingly, the removals of nitrogen, phosphorus, and COD in the SRT-15d and SRT-30d reactors changed obviously with dosing of aluminum salt. At the stage 2, although phosphorus and COD removals were enhanced with the dosing of aluminum salt, the effluent concentration of NH4+-N was obviously increased. Interestingly, the increase of NH4+-N concentration in effluent in the SRT-15d reactor (91.80%) was lower than that of the SRT-30d reactor (111.84%) which meant the inhibition of nitrification in the SRT-15d reactor was lower than the SRT-30d reactor. On the one hand, the shorter SRT could induce the sludge growth rate with better bacterial activity. Xu et al. (2015) pointed out that the bacterial growth rate was obviously improved with the increase of high organic loading at the shorter SRT. The MBR with a shorter SRT (SRT = 15d) led to higher EPS (Yu and Su, 2012). On the other hand, the shorter SRT might cause the change of sludge growth due to the decline of accumulated aluminum in the sludge. Because more sludge was discharged at a shorter SRT, more aluminum salt was also excreted (He et al., 2016). However, the quantitative relationship between the accumulated aluminum salts and the bacterial activity was still lacking.
Fig. 2. Influence of the accumulated aluminum on the observed sludge yield at different SRTs (SRT15-1: stage1, SRT = 15 d and without dosing of aluminum. SRT15-2: stage2, SRT = 15 d and with dosing of 20 mg Al L−1. SRT30-1: stage1, SRT = 30 d and without dosing of aluminum. SRT30-2: stage2, SRT = 30 d and with dosing of 20 mg Al L−1).
MLSS and MLVSS in SRT-15d reactor were increased from 2.36 g SS L−1 and 1.53 g VSS L−1 at stage 1–3.32 g SS L−1 and 2.08 g VSS L−1 at stage 2. Meanwhile, those in SRT-30d reactor were correspondingly increased from 2.62 g SS L−1 and 1.61 g VSS L−1 to 3.90 g SS L−1 and 2.40 g VSS L−1. At the stage 1, the observed sludge yields were 0.203 ± 0.010 and 0.156 ± 0.006 g VSS g−1 COD at the SRT-15d and SRT-30d reactors, respectively. After dosing of aluminum salt, the observed sludge yields correspondingly decreased to 0.185 ± 0.042 and 0.119 ± 0.051 g VSS g−1 COD, respectively. The short SRT could enhance the microorganisms renew and uptake organic compounds to synthesize new bacterial cell (Corsino et al., 2019). Furthermore, the accumulated aluminum suppressed the growth yield of sludge at the stage 2 and the decline of sludge yield at the SRT-15d reactor (8.9%) was lower than that of the SRT-30d reactor (23.7%). As shown in Fig. 3(a), it reflected the accumulation of aluminum salt in sludge of the SRT-15d and SRT-30d reactors. By theoretical calculation, the amount of accumulated aluminum in the SRT-15d and SRT-30d reactors were 130.87 and 156.94 mg Al g−1 SS, respectively, at the end of stage 2. With the mechanisms of charge neutralization, polymer bridging and electrostatic patch (Chai et al., 2014), most aluminum salts could accumulate in sludge and accumulated aluminum might bring negative effects on bacteria. As shown in Fig. 3(b), by calculation, the amount of accumulated aluminum in the SRT-15d reactor (230.59 mg Al g−1 VSS) was lower than that of the SRT-30d reactor (269.92 mg Al g−1 VSS) at the end of stage 2. It could be expected that the equilibrium accumulative concentration of aluminum salt would be achieved sooner at the SRT-15d reactor. As calculated, the equilibrium accumulative concentration of aluminum would be achieved on the 64th day (after dosing aluminum salt) in the SRT-15d reactor, while the SRT-30d reactor would reach the equilibrium value on the 107th day (after dosing aluminum salt). The expected
3.3. Analysis of mass balance at different SRTs The influence of the accumulated aluminum on the observed sludge yield at different SRTs is shown in Fig. 2. The average concentrations of 4
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Fig. 4. Influence of accumulated aluminum on the activity of AOB and NOB at different SRTs (SRT15-1: stage1, SRT = 15 d and without dosing of aluminum. SRT15-2: stage2, SRT = 15 d and with dosing of 20 mg Al L−1. SRT30-1: stage1, SRT = 30 d and without dosing of aluminum. SRT30-2: stage2, SRT = 30 d and with dosing of 20 mg Al L−1).
significant inhibition of aluminum salt on nitrification. Chen et al. (2019) reported that the high concentration of dosing metal ions (5 mg Cu2+ L−1 and 60 mg Fe3+ L−1) would weaken enzyme activities. Furthermore, because the ammonia monooxygenase of AOB affected more easily, the AOB was more suppressed severely than NOB due to the accumulated aluminum (Yang et al., 2019). The shorter SRT was prone to weaken the inhibitory effect of accumulated aluminum salt on the AOB and NOB. Liu et al. (2011) showed the inhibition of accumulated aluminum salt was more serious on the AOB with the long-term dosing of 56 mg Al L−1, and the nitrification could even decline to 20% without discharging aluminum salt. With increased metal salts dosed into sludge, the ammonia oxidation rates showed a downward trend (Chen et al., 2019). These findings indicated that the higher concentration of aluminum salt in biomass would significantly enhance the suppressing effect on activity of AOB and NOB. Thus, the reduction of accumulated aluminum salt in sludge would be the key step to recover the biological activity. In this study, controlling SRT could effectively reduce the accumulation of aluminum in sludge, and the inhibition effect of aluminum on activity of AOB and NOB was weakened at shorter SRT (in SRT-15d reactor). Therefore, controlling shorter SRT could be employed to alleviate the suppressing effect on nitrification by accumulated aluminum salt in the A/O-MBR.
Fig. 3. Effect of different SRTs on accumulation of aluminum in the sludge. (a) Aluminum salt in MLSS of sludge, (b) Aluminum salt in MLVSS of sludge.
equilibrium concentrations of accumulated aluminum in the sludge were 289.2 and 489.6 mg Al g−1 VSS in the SRT-15d and SRT-30d reactors, respectively. He et al. (2016) showed that the sludge structure and properties would become worse under high salinity environment (> 1 wt% salinity). Besides, it was confirmed that high concentration of aluminum salt covering on sludge would compress the sludge EPS (Cao et al., 2016), which also influenced the bacterial activities with the increasing accmulated aluminum. The high concentration of metal salt would exceed the cell surface capacity (related to the EPS capacity), even entered into the cells to cause the death of bacteria (Ma et al., 2018). Due to the higher growth yield of sludge and lower rate of aluminum accumulation in the shorter SRT (SRT = 15 d) reactor, the accumulated aluminum salt in the sludge would be controlled at a relatively low level. Therefore, controlling shorter SRT could be used to alleviate inhibition of nitrification by accumulated aluminum salt in the A/O-MBR, but the expected result required balancing the sludge growth rate with the aluminum accumulation rate.
3.5. Influence of accumulated aluminum on the activity of biological phosphorus removal and denitrification of A/O-MBRs at two different SRTs The effect of accumulation of aluminum salt on denitrification activity of sludge is shown in Fig. S2. Without dosing of PAC, the NOx−-N removal rates were 67.04% and 26.20%, respectively for the SRT-15d and SRT-30d reactors at 30 min. With accumulation of aluminum, the NOx−-N removals obviously increased to 82.31% and 42.19%, respectively in the SRT-15d and SRT-30d reactors. After dosing of aluminum salt, the denitrification activity of sludge in the both reactors were improved. The improvement of sludge denitrification activity in a longer SRT (SRT = 30 d) was more obvious which was caused by the denser flocculating structure to provide anaerobic environment in MBRs with a higher concentration of aluminum salt (Cao et al., 2016). As reported the metal ions were also the important factor to the specific reductase biosynthesis of sludge denitrification activity (Zhu et al., 2013). Although it was still no clear why NUR was improved with accumulation of aluminum salt, the denitrification activity of sludge at
3.4. Influence of accumulated aluminum on the activity of AOB and NOB in A/O-MBRs at two different SRTs The influence of accumulated aluminum on the activity of AOB and NOB at different SRTs is shown in Fig. 4. With the accumulation of aluminum salt in the reactor, the activity of AOB was decreased by 4.4% and 10.9% at the SRT-15d and SRT-30d reactors respectively (p = 0.27 and 0.04, respectively, p value is significant compared to the activity of AOB after aluminum accumulation in the SRT-30d reactor), while the activity of NOB was decreased by 18.3% and 23.9%, respectively (p = 0.07 and 5.18 × 10−4, respectively, p values are significant compared to the activity of NOB after aluminum accumulation in the SRT-15d and SRT-30d reactor). After dosing of PAC, it indicated a 5
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the shorter SRT (SRT = 15 d) was still much higher than that at the longer SRT (SRT = 30 d). Thus, controlling shorter SRTs would be beneficial to denitrification with the accumulation of aluminum salt. The dosing of PAC could chemically reduce phosphorus concentration in effluent, whereas it also influenced biological phosphorus removal. The profile of phosphorus release and uptake with and without accumulation of aluminum at different SRTs is shown as Fig. S3. Before dosing of PAC, the phosphorus release concentrations were 14.15 and 17.50 mg L−1, respectively in the SRT-15d and SRT-30d reactors at 150 min. After dosing of aluminum salt, they decreased to 11.72 and 13.60 mg L−1, respectively in the SRT-15d and SRT-30d reactors. It indicated that the sludge with accumulated aluminum salt showed a suppressing effect on phosphorus removal. Liu et al. (2016) described the phosphorus adoption in aluminum-sludge and it might refer to the role of neutralization and bio-chemical bridging (Cui et al., 2018). The incomplete phosphorus release also occurred in our experiment which could be explained as the adsorption of aluminum salt. Still, controlling the shorter SRT (SRT = 15 d) could weaken the suppressing effect of phosphorus release. Also, in the process of phosphorus uptake, it showed that before dosing of PAC, the phosphorus uptake rate were 20.80% and 29.93%, respectively for the SRT-15d and SRT30d reactors at 270 min. However, after accumulation of aluminum salt, the phosphorus uptake rates were 95.80% and 94.43%, respectively in the SRT-15d and SRT-30d reactors. It reflected that the accumulated aluminum salt also had a suppressing effect on phosphorus uptake. Although the biological activity of phosphorus removal was suppressed by accumulated salt, the A/O-MBR with dosing of aluminum salt showed an excellent performance on chemical phosphorus removal.
inappropriately influence our work, there is no commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was funded by the National Natural Science Foundation of China (51878232) and Foundation of Anhui Shun Yu Water Co., Ltd. (W2018JSKF0137). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ibiod.2019.104755. References APHA, 2012. Standard methods for the examination of water and wastewater. Washington, D.C 21 st Edition. Cao, B., Zhang, W., Wang, Q., Huang, Y., Meng, C., Wang, D., 2016. Wastewater sludge dewaterability enhancement using hydroxyl aluminum conditioning: role of aluminum speciation. Water Res. 105, 615–624. Chai, S.L., Robinson, J., Mei, F.C., 2014. A review on application of flocculants in wastewater treatment. Process Saf. Environ. 92, 489–508. Chen, Y., Wang, Q., Zhao, S., Yang, W., Wang, H., Jia, W., 2019. Removal of nutrients and emission of nitrous oxide during simultaneous nitrification, denitrification and phosphorus removal process with metal ions addition. Int. Biodeterior. Biodegrad. 142, 143–150. Chen, Y., Wu, Y., Wang, D., Li, H., Wang, Q., Liu, Y., Peng, L., Yang, Q., Li, X., Zeng, G., Chen, Y., 2018. Understanding the mechanisms of how poly aluminium chloride inhibits short-chain fatty acids production from anaerobic fermentation of waste activated sludge. Chem. Eng. J. 334, 1351–1360. Chon, D.H., Rome, M., Kim, Y.M., Park, K.Y., Park, C., 2011. Investigation of the sludge reduction mechanism in the anaerobic side-stream reactor process using several control biological wastewater treatment processes. Water Res. 45, 6021–6029. Corsino, S.F., Capodici, M., Di Pippo, F., Tandoi, V., Torregrossa, M., 2019. Comparison between kinetics of autochthonous marine bacteria in activated sludge and granular sludge systems at different salinity and SRTs. Water Res. 148, 425–437. Cui, X., Huo, M., Chen, C., Yu, Z., Zhou, C., Li, A., Qiao, B., Zhou, D., Crittenden, J.C., 2018. Low concentrations of Al(III) accelerate the formation of biofilm: multiple effects of hormesis and flocculation. Sci. Total Environ. 634, 516–524. Falahti-Marvast, H., Karimi-Jashni, A., 2015. Performance of simultaneous organic and nutrient removal in a pilot scale anaerobic–anoxic–oxic membrane bioreactor system treating municipal wastewater with a high nutrient mass ratio. Int. Biodeterior. Biodegrad. 104, 363–370. Fan, J., Jin, Z., 2009. Water Quality Engineering, 1 st Edition. China Architecture & Building Press, Beijing, pp. 352–353. Friedrich, M., Takács, I., Tränckner, J., 2015. Physiological adaptation of growth kinetics in activated sludge. Water Res. 85, 22–30. Han, X., Zhou, Z., Mei, X., Ma, Y., Xie, Z., 2018. Influence of fermentation liquid from waste activated sludge on anoxic/oxic- membrane bioreactor performance: nitrogen removal, membrane fouling and microbial community. Bioresour. Technol. 250, 699–707. Hass, C.N., 1979. Oxygen uptake rate as an activated sludge control parameter. J. Water Pollut. Control Fed. 51, 938–943. He, H., Chen, Y., Xiang, L., Yan, C., Yang, C., Zeng, G., 2016. Influence of salinity on microorganisms in activated sludge processes: a review. Int. Biodeterior. Biodegrad. 119, 520–527. Ji, B., Kai, Y., Wang, H., 2015. Impacts of poly-aluminum chloride addition on activated sludge and the treatment efficiency of SBR. Desalin. Water Treat. 54, 2376–2381. Kochkodan, V., Hilal, N., 2015. A comprehensive review on surface modified polymer membranes for biofouling mitigation. Desalination 356, 187–207. Li, R.H., Wang, X.M., Li, X.Y., 2018. A membrane bioreactor with iron dosing and acidogenic co-fermentation for enhanced phosphorus removal and recovery in wastewater treatment. Water Res. 129, 402–412. Liu, R., Zhao, Y., Sibille, C., Ren, B., 2016. Evaluation of natural organic matter release from alum sludge reuse in wastewater treatment and its role in P adsorption. Chem. Eng. J. 302, 120–127. Liu, Y., Shi, H., Li, W., Hou, Y., He, M., 2011. Inhibition of chemical dose in biological phosphorus and nitrogen removal in simultaneous chemical precipitation for phosphorus removal. Bioresour. Technol. 102, 4008–4012. Londono, S.C., Hartnett, H.E., Williams, L.B., 2017. Antibacterial activity of aluminum in clay from the colombian amazon. Environ. Sci. Technol. 51, 2401–2408. Ma, B., Li, Z., Wang, S., Liu, Z., Li, S., She, Z., Yu, N., Zhao, C., Jin, C., Zhao, Y., Guo, L., Gao, M., 2019. Insights into the effect of nickel (Ni(II)) on the performance, microbial enzymatic activity and extracellular polymeric substances of activated sludge. Environ. Pollut. 251, 81–89. Ma, Y., Yuan, D., Mu, B., Zhou, J., Zhang, X., 2018. Reactor performance, biofilm property and microbial community of anaerobic ammonia-oxidizing bacteria under longterm exposure to elevated Cu (II). Int. Biodeterior. Biodegrad. 129, 156–162.
3.6. Practical implications In this study, with dosing of PAC, the increasing rate of TMP and the effluent concentration of phosphorus obviously decreased with the dosing of PAC which indicated that it could enhance phosphorus removal and effectively reduce membrane fouling (Fig. S3 and Fig. S4). Also, the denitrification rates of the SRT-15d and SRT-30d reactors were improved with the accumulation of aluminum. The denitrification activity of sludge at the shorter SRT (SRT = 15 d) was also much higher than that of the longer SRT (SRT = 30 d). Although the accumulated aluminum salts would inhibit the activity of AOB and NOB, the dosing of PAC could bring some benefits on the performance of A/O-MBRs. Controlling shorter SRT could alleviate inhibition of nitration and nitrosation in the A/O-MBRs. At the same time, some researchers had developed new pathways for recycling aluminum-containing sludge (Liu et al., 2016), and it was more potential to be reused as an emerged substrates in constructed wetland for enhancing phosphorus removal (Yang et al., 2018). Therefore, controlling SRT can be used as an important tool to improve the performance of A/O-MBRs with dosing of PAC. 4. Conclusion The main conclusions could be drawn as follows: (1) The inhibition rate of aluminum salt on SOUR was positively correlated with the content of aluminum salt in sludge. (2) The accumulated amount and rate of aluminum salt of sludge at the shorter SRT was lower than that of the longer SRT. (3) Controlling shorter SRT could be employed as an effective tool to alleviate inhibition of nitrification by accumulated aluminum in A/ O-MBR. Conflicts of interest statement Our all the authors declare that we have no financial and personal relationships with other people or organizations that can 6
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